Electrostatic powder coating process

ABSTRACT

A process of electrostatically coating cylindrical metal objects such as pipes with powdered resins is provided, in which the object is shot blasted, then subjected to high pressure fluid spray, e.g. water or steam jets, then heated above the fusion temperature of the resin, polarized and the resin powder sprayed on.

United States Patent Shaw et a1. 1 1 Sept. 9, 1975 154] ELECTROSTATIC POWDER COATING 832.288 10/1906 Benncy 117 49 PROCESS 2.380.738 7/1945 Eppler 117 49 2.387.193 10 1945 Swenarton 51/321 1 lnvfirltorsl Leslie Earl Shaw. 2.528.787 11/1950 Roland 117 49 Woodbridge, Ontario; Harold 2605,5961 8/1952 Uhri 51/321 Francis Jarvis, R,R Nu Mofful 2985,0511 5/1961 Schwacha 1 51/321 Ontario both f Canada 3.294.573 12/1966 Michael ct a1. 117/49 3,502,492 3/1970 Spiller 117 17 1 Filed: ov. 12,1973 3.524.367 8/1970 Franz 51 321 3563785 2/1971 Oga ct a1 117/49 [21] 10414924 3581922 6/1971 Versoy 117 94 Rdated U 5 Appncation m 3.608.249 9/1971 Sullivan 51/321 3.664.888 5/1972 Ogfl ct 211. 117 49 [63] Continuation-impart of Scr. No. 211,721 Dec. 23,

1971. abandoned.

[52] U.S. Cl. 427/29; 427/32; 427/134; 427/290; 427/315; 118/621; 118/629 [51] Int. C1. B05B 5/02 [58] Field OfSearCh 1.117/17,21,49,18; 118/621, 624, 627, 629. 73', 51/319, 321', 427/29. 32, 184 290, 315

[561 References Cited UNITED STATES PATENTS 7531111) 3/1904 McDowell 117/49 Primary E.raminerMichae1 Sofocleous 57 ABSTRACT A process of electrostatically coating cylindrical metal objects such as pipes with powdered resins is provided. in which the object is shot blasted, then subjected to high pressure fluid spray. e.g1 water or steam jets. then heated above the fusion temperature of the resin, polarized and the resin powder sprayed on.

10 Claims, 2 Drawing Figures ELECTROSTATIC POWDER COATING PROCESS CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 211.721 filed Dec. 23. 1971. and now abandoned.

FIELD OF THE INVENTION This invention relates to methods of coating metal objects with powdered materials. More particularly. it relates to methods of coating steel pipes and pipe scctions with powdered resins. such as epoxy resins.

DESCRIPTION OF THE PRIOR ART The provision of thermally and/or electrically insulating coatings of epoxy resins on steel pipe sections. which are intended to be used to form fluid conducting pipelines. either above or below the ground. is known. Customarily. the process involves the spraying of the epoxy resin in powder form onto steel pipe electrostatically. After abrasive cleaning of the pipe surface. e.g. by shot blasting. the metal pipe is heated. given an electrical charge. and epoxy resin powder charged oppositely to the pipe is applied to the pipe by spraying, so that the powder adheres electrostatieally to the pipe surface. The coating cures on the hot pipe thereon. The epoxy resin powder which does not adhere to the pipe surface after spraying is collected and returned to the epoxy powder storage hopper to be recycled.

There are certain problems associated with these prior art processes. For example. it is very difficult to obtain a uniform thickness of resin coating. To ensure adequate thickness of coating at all locations on the pipe surface. excessive amounts of epoxy resin must be applied. due to uneven application. Also. the necessary abrasive cleaning operation appears to leave small slivers of metal protruding radially from the exterior pipe surface. which are likely to protrude through the applied resin coating. and spoil the continuous nature of the applied insulation.

The abrasive cleaning. normally shot blasting with steel shot. is necessary not only for its cleaning function. but also in order to provide an anchor pattern of pits. ridges. etc.. on the steel pipe surface. to assist in proper adhesion of the resin coating thereto. Any electrostatic powder coating process requires that the sur face to be coated be very clean prior to powder application.

SUMMARY OF THE INVENTION It has now been found that improved resinous coatings can be electrostatically applied to metal objects by a process which involves the steps of shot blasting the metal object. subjecting the metal object to a high pressure fluid stream washing operation. polarizing the object. spraying the object with charged powdered resin so as electrostatically to coat the object with the resin. and curing the resin thereon.

The use of a high pressure fluid stream washing system after the abrasive cleaning system is surprisingly superior to any other method we have found. for cleaning the metal surface. in terms of quality of finished products. As previously mentioned. it is important for a metal surface to be very clean prior to electrostatic powder coating ofthe surface. The abrasive cleaning of the metal is best performed by shot blasting. for efficicncy of cleaning and to apply a suitable anchor pattern. The shot blasting removes rust. protruding metal slivers. gross surface irregularities etc.. which inevitably are formed during manufacture of large steel objects such as pipes. In doing so, however. the abrasion effect apparently establishes an electrostatic field on the pipe surface. which consequently causes some of the dust generated by the shot blasting to cling to the pipe surface. It also causes any residual metal slivers on the surface to stand up.

The problem at this stage is to remove the clinging dust. etc.. to provide a clean surface for electrostatic powder coating. There are many ways. theoretically. in which this could be done. e.g. by wiping, brushing. light grinding. etc.. but the method which has been found to be by far the best, in terms of the quality of the end product. is high pressure fluid jets. e.g. water or steam jets. This is despite the fact that application of water or steam to a newly cleaned steel surface which is about to be heated would be expected to result in corrosion. and expensive steps have just been taken for thorough removal of corrosion from the surface. Nevertheless. use of high pressure water or steam washing at this stage leads to surprising and unpredictable increases in the quality of the final products. both in terms of uni formity of the final coating and in terms of adhesion of the coating to the metal surface. It also appears that the high pressure water or steam washing at this stage has the effect of depolarizing the metal object, i.e. removal ofelectrostatic charges imparted thereto by the friction of the abrasive cleaning process. Then when the object is subsequently polarized ready for electrostatic spraying of resin powder. the applied charge is of known magnitude and even distribution. so as to improve uni formity of the coating applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a further aspect. this invention may also include a second electrostatic powder spraying step. utilizing powdered resin from the first spraying application which falls off or does not contact the objects surface (overspray material). This may comprise of the order of 10 to 15% of the material sprayed. Conventionally. such overspray material has been collected and reused in admixture with the fresh powdered resin. from the same spray station. It has been found. however. that it is better to spray this overspray material onto the pipe in a second and subsequent electrostatic spray operation. and avoid mixing it with the new mate rial. Control of the first spray operation is much easier in the absence of overspray material. which may well have residual electrostatic charge. uneven particle size and have agglomerated as a result of subjection to the first electrostatic spray operation.

The process of this invention has been primarily developed in connection with the application of powdered epoxy resin electrostatically to the external sue faces of steel pipes, and will be further described with specific reference thereto. However. it will be appreciated that it is of wider application. and can be applied to other types of metal objects. and to other types of powdered resins. which are capable of electrostatic spray application to the metal object. Other resins ca pablc of application electrostatically in powder form include vinyl polymers. polyethylene, polyesters, cellulose, nylon, and fluorocarbon polymers.

The process of the invention will be further described with reference to the accompanying drawings. in which:

FIG. 1 is a diagrammatic plan view of an apparatus for conducting the process of the invention;

FIG. 2 is a perspective view. partly cut away. of the powder spray chamber used in the process of the invention.

In the drawings, like reference numerals indicate like parts.

Referring to FIG. 1 of the drawings, a cylindrical steel pipe is axially advanced continuously, and with continuous rotation about its longitudinal axis. past a number of stationary treatment stations. Such advancement and rotation means may comprise first and second pipe tracks 11 and 12 defined by a plurality of pipe supporting rollers or wheels. each with its axis inclined to the direction of axial travel of the pipe 10, so that by rotating such rollers or wheels the pipe is simultaneously advanced and rotated. Such a track system is disclosed in Canadian Pat. No. ol7,049. issued Mar. 28, 196] to Leslie Shaw. In the alternative the rollers or wheels of such a track system may be replaced by chain and spx cket systems inclined to the direction of axial travel of the pipe. With this alternative arrangement. pairs of sprocket wheels are provided, one member of each pair on each side of the track, and the endless chain extending between them. so that the chains. inclined to the direction of axial travel. form cradles supporting, advancing and rotating the pipe.

As it is advanced and rotated in this manner along the first pipe track 1 l, the pipe 10 passes a cleaning station 13. where it is abrasively cleaned. by shot blasting in the well known way. The speed of travel and rotation of the pipe 10 is adjusted to make sure that the entire exterior surface thereof is thoroughly cleaned from one or more shot blasting heads.

When the pipe 10 has travelled clear of the cleaning station 13 it is ejected from the first pipe track 11, by suitable arrangement of the track itself, onto a gently sloping ramp 14. The ramp 14 provides facilities for visual inspection of the pipes. and rejection of those not satisfactorily cleaned at 13. The pipe 10 rolls down ramp 14 and is received by the second pipe track 12. The pipe 10 is advanced and rotated by the second pipe track 12 to a high pressure water spray station 15. where the entire cleaned exterior surface of the pipe 10 is subjected to high pressure water spray. The pressure of the spray is preferably from about l000 to about 5000 psi. most preferably about 3000 psi. and in any case high enough to remove dust particles left on the surface by the shot blasting operation. As shown the spray is effected from a plurality of stationary nozzles. Alternatively a ring spray device could be provided through the centre of which the cleaned pipe passes.

Next. the pipe 10 is advanced and rotated past an air drying station 16. where cold air jets impinge upon the pipe to remove residual water from the high pressure washing. it is desirable to ensure complete drying ol'thc pipe prior to heating and application of powdered resin, so as to avoid risk of local corrosion on the pipe. which will adversely affect adherence of the coating to the pipe. The air jets in drying station 16 operate at a pressure of from about to 200 psig. preferably at about 100 psig.

Then the pipe 10 passes into and is conveyed through a gas fired oven 17 where it is heated to a suitable temperature so that the resin subsequently applied to the surface thereof will be raised to a sufficient temperature to fuse and to commence to cure. in the case of the preferred epoxy resin. this temperature is of the order of 450 to 500F. suitably about 475F.

The gas fired oven 17 has to be of substantial length in order to handle pipes of up to 50 ft. in length. which are coated in accordance with the present process. In order to convey the pipes through oven 17, they are supported upon inclined track wheels comprising a rubber pneumatic tire, around the periphery of which are affixed chains. A steel plate is welded to the outer surface of the chains, so that the periphery of the tire is overlaid successively by chains and the steel plate. The plate is provided with a depending deflection skirt to guide the end of the pipe into supporting engagement with these track wheels. The wheels protrude downwardly below the oven floor, and dip in troughs of cold water. The chain and plate arrangement allows for air circulation around the tire. to assist in its cooling. Steel surfaced support wheels at this location are undesirable, because of the risk that they will destroy the anchor pattern applied by the shot blasting.

The hot pipe 10 is then electrostatically polarized by application thereto of the necessary electric charge. Then it passes. continuously and with continuous rotation. through a first electrostatic powder spray chamber 18 containing spray heads 19, from which pow dered primer resin. electrostatically charged, is sprayed onto the charged exterior surface of the pipe 10.

Then the primed pipe 10 is fed. continuously and with continuous rotation, through a second electrostatic powder spray chamber 20, where the top coat of powdered resin is applied electrostatically from spray heads 21 to the hot primed pipe 10. The pipe 10 then passes through a curing station comprising an oven 22 containing infra-red irradiation heaters 23. It will not always be necessary to use the heaters 23, since in some cases. depending upon the nature of the resin being sprayed and the temperature of the pipe. curing will take place reasonably quickly without further application of heat.

After the completion of curing, whether involving heating or not, it is desirable that the coated pipe be cooled so that it can be handled easily without undue delay. For this purpose, the coated pipe 10 is then conveyed. continuously and with continuous rotation, through a quench trough 24, where it is cooled by low pressure cold water sprays.

From the quench trough 24, the pipe 10 passes through an electrical probe 25. where it encounters electrical brushes arranged to trail on the surface of the coated pipe. If a hole in the coating is encountered. the probe gives an electrical signal so that if necessary the hole can be repaired. Then the pipe 10 is conveyed away from the process apparatus. to storage.

The powder spray chamber 18. 20 used for primer application and top coat application respectively are shown in more detail in FIG. 2. The chamber comprises a housing 26, of metal. of part cylindrical form, provided with a part cylindrical wall 27, an apertured planar rear wall 28 and apertured planar end walls 29, 30. The pipe [0 to be coated enters through a circular aperture in end wall 30 and departs through a circular ap erturc in end wall 29.

The rear wall 28 of the housing 26 is provided with a rectangular aperture 31 in which the spray guns 2] are mounted to spray resin powder on the pipe 10. The spray guns 2] are adjustably clamped to a transverse bar 32, itself vertically adjustable on a frame 33, so that the positions and spray angles of the spray guns 2] can be varied at will.

An outlet pipe 34 communicates with the housing 26 near one bottom corner thereof. attached to a vacuum pump. through which overspray material is removed. A circulation of air within the housing 26 is arranged in a generally clockwise direction (viewing end wall 29) towards outlet 34, to remove the overspray powder. after the manner of a fluidized bed. as follows. A perforated baffle plate 35 is located below the aperture 3] in rear wall 28. Low pressure air is supplied continuously to the plate 35 through air duct 36. Thus air issues from the perforated baffle plate 35 in an upward direction towards the spray guns 21. thereby in conjunction with the vacuum drawn outlet 34 establishing an air flow in the aforementioned clockwise direction around the pipe 10. The direction of spray of powder from the spray guns 21 as shown contributes to this air flow pattern. The air flow pattern can be regulated by adjusting the angle of the plate 35 which is pivotablc about a horizontal axis through its approximate mid point. at 37. Further adjustment is provided by a wedge shaped damper 38 pivotally mounted in an apertured plate 39. to overlie outlet 34. The plate 39 is mounted obliquely across the corner of the housing 26 where outlet 34 is located. but extending axially the length of the housing 26. The aperture (or cut-away portion) 40 of the plate 39 is located downwardly, and wedge shaped damper 38 is pivotally mounted so as to cover cut-away portion 40 to a greater or lesser extent. with its larger end over the outlet 34. Additional control of the air flow within the housing 26 is thus provided. by pivotal adjustment of damper 38.

There is provided a part circular shield 41 in the space between the rear wall 28 of the housing 26 and the plate 39. to prevent the creation of a dead space at the junction between these members.

The part cylindrical wall 27 of the housing 26 is provided with an inner liner 42. of porous plastic. cg porous polyethylene. A small gap is left between the liner 42 and the wall 27. Air inlet ports 43, 44 are provided in the wall 27. through which cold air is continuously introduced between the liner 42 and the wall 27. By this means a continuous flow of air is maintained over the surface of the liner 42. thereby preventing deposit of spray powder on the liner and the housing. and preventing overheating of the liner and housing as a result of radiation from the hot pipe. The porous liner 42 also ensures even air flows and avoids high velocity flow pockets.

It will be noted that in the preferred embodiment of the process as illustrated, all the steps after the pipe 10 is received on track 12 are arranged to be conducted in line. This greatly facilitates conducting the process continuously. with the pipe l0 moving continuously and with continuous rotation through the process. Long sections ofpipe. for example 40 feet in length. are handled continuously in this preferred process.

Whilst one preferred embodiment of the process has been described with reference to the drawings. there are many variations which can be effected. in some cases dependent upon the type of resin being used and its curing characteristics.

For example. many resins do not require the prior application of a primer. In such cases. the two powder spray stations may still be used. the second being fed with overspray material from the first. as previously described. Where a primer is used. there may be a third or a fourth spray station for spraying with overspray material from either or both of the primer application and the top coat application.

The hot coated pipe should preferably be handled carefully until its curing has been completed. In some cases the use of the heating oven 22 is not necessary. as previously mentioned. In such case prior heating of the pipe effects cure of the resin. but then particular care must be taken to avoid damaging the coating as it cools and cures. To achieve this, and to convey the pipe away from the apparatus to allow continuous running of the process. suitably the cooling pipe is moved continuously and with continuous rotation by supporting it on driven track of inclined wheels. as previously described. but in which the supporting surfaces of the wheels are of rubber, and the wheels are about half submerged in cold water. It is desirable to use such wheels at all locations where hot pipe is to be supported and conveyed. The provision of a cold water bath into which each wheel dips serves to prevent overheating of the wheel and destruction of the rubber friction surfaces thereon. Further. the arrangement serves to apply a film of cold water between the wheels and the hot. uncured coating of the pipe, thereby preventing sticking of the coating to the rubber. and perhaps causing a degree of shock cooling on the outermost layer of the coating. forming a skin of cured outermost material to protect the inner layers as they eure.

As a further feature of the invention. it is desirable to provide a heat barrier at each end of the pipe section. during transportation through the process. This can be in the form of a solid plug inserted into the end of the pipe. In a preferred embodiment ofthe process. where a number of pipe sections are conveyed successively through the process. a supporting trolley is provided which supports adjacent ends of successive pipe sections being conveyed through the process. as described in U.S. Pat. No. 3.743.124 Harold F. Jarvis. issued July 3. 1973. Such trolleys are provided with solid inserts which enter the pipe ends and support the pipe. The solid inserts provide heat barriers, lessening the flow of heat away from a coated pipe section and thereby facilitating heat control and slowing the rate of cooling of the pipe sections.

In a specific example conducted to demonstrate the beneficial effects obtainable from a process in accordance with the invention. comparative tests were run. First. a section of steel pipe of length 40 feet and external diameter 18 inches was electrostatically coated with powdered epoxy resin by a process according to the prior art. This involved the step of shot blasting the pipe exterior for cleaning purposes. heating the pipe to 475F in a gas tired oven (the temperature being determined by means of an optical pyrometer). polarizing it. coating it with powdered epoxy resin in one operation. with recycle of overspray material to the same spray gun. intended to provide a coating of thickness ll) to 12 mils. and allowing the coated pipe to cool slowly to cure the epoxy resin. The process was conducted on a continuously advancing and rotating pipe section.

The coating was then tested by moving an electrical probe over the coated pipe surface, so that a potential difference of about 100 volts was applied across the coating. Where the coating was deficient in thickness, sparking (jeeping") occurred, and these jeeps were counted.

The pipe coated in this manner gave about 80 jeeps over the 40 foot length of pipe.

Then another similar pipe section was coated with the same epoxy resin powder to the same intended thickness and by the same method. except that after the shot blasting step the pipe was subjected, all over its exterior surface, to a water spray at a pressure of about 3000 psi, and the overspray material was coated on the pipe from a second spray gun located downstream from the first.

Under the same test conditions, this coated pipe gave less than l0 jeeps over the 40 foot length of pipe.

What we claim as our invention is:

1. An electrostatic process of coating a cylindrical meta] object with powdered resin which includes the steps of shot blasting the object. causing the object to rotate about its axis. and whilst maintaining rotation of the object, sequentially subjecting the object to a high pressure water or steam spray, removing any residual moisture from the object. heating the object to a temperature which will fuse the powdered resin subsequently applied, polarizing the object. electrostatically spray coating the object with powdered resin and allowing the coated object to cool.

2. The process of claim I, wherein the high pressure spray is at a pressure of from about 1,000 to about 5,000 psi.

3. The process of claim 2 wherein the object is a steel pipe.

4. The process ot'claim 2 wherein the powdered resin material is an epoxy resin.

5. The process of claim I. which also comprises feeding overspray powdered resin material from the spray coating step to a second spray coating station from which it is eleetrostatically spray coated onto the ob ject.

6. The process of claim 1 wherein the powdered resin material is an epoxy resin.

7. The process of claim 6 wherein the object is a steel pipe, the pipe being heated to a temperature of from about 450F to about 500F prior to coating.

8. The process of claim 7 in which the step of rotating the pipe is accompanied by the step of advancing the rotating pipe so as to move it past a stationary cleaning station at which the shot blasting takes place, a water or steam spray station at which the pipe is sprayed, a heating station at which the pipe is heated and a coating station at which the pipe is coated.

9. The process of claim 8 in which the step of rotating and advancing the pipe is carried out by supporting the pipe on pairs of driven supporting wheels or rollers defining a trap for the pipe extending through said stations. the axis of said wheels or rollers being inclined to the direction of axial travel of the pipe, said wheels or rollers having friction surfaces which engage the pipe, and dipping into cold water as they rotate.

10. The process of claim 8, including the step of providing the pipe section with a heat barrier in the form ofa solid plug inserted into the end thereof, prior to the rotation and advancement step. 

1. An electrostatic process of coating a cylindrical metal object with powdered resin which includes the steps of shot blasting the object, causing the object to rotate about its axis, and whilst maintaining rotation of the object, sequentially subjecting the object to a high pressure water or steam spray, removing any residual moisture from the object, heating the object to a temperature which will fuse the powdered resin subsequently applied, polarizing the object, electrostatically spray coating the object with powdered resin and allowing the coated object to cool.
 2. The process of claim 1, wherein the high pressure spray is at a pressure of from about 1,000 to about 5,000 psi.
 3. The process of claim 2 wherein the object is a steel pipe.
 4. The process of claim 2 wherein the powdered resin material is an epoxy resin.
 5. The process of claim 1, which also comprises feeding overspray powdered resin material from the spray coating step to a second spray coating station from which it is electrostatically spray coated onto the object.
 6. The process of claim 1 wherein the powdered resin material is an epoxy resin.
 7. The process of claim 6 wherein the object is a steel pipe, the pipe being heated to a temperature of from about 450*F to about 500*F prior to coating.
 8. The process of claim 7 in which the step of rotating the pipe is accompanied by the step of advancing the rotating pipe so as to move it past a stationary cleaning station at which the shot blasting takes place, a water or steam spray station at which the pipe is sprayed, a heating station at which the pipe is heated and a coating station at which the pipe is coated.
 9. The process of claim 8 in which the step of rotating and advancing the pipe is carried out by supporting the pipe on pairs of driven supporting wheels or rollers defining a trap for the pipe extending through said stations, the axis of said wheels or rollers being inclined to the direction of axial travel of the pipe, said wheels or rollers having friction surfaces which engage the pipe, and dipping into cold water as they rotate.
 10. The process of claim 8, including the step of providing the pipe section with a heat barrier in the form of a solid plug inserted into the end thereof, prior to the rotation and advancement step. 